多级压裂水泥环界面密封失效数值模拟

Numerical Simulation of Interface Seal Failure of Cement Sheath during Multi-stage Fracturing

四川地区页岩气开发过程中环空带压现象严重,严重影响页岩气井的高效开发,因此准确了解页岩气井多级压裂过程中固井界面微环隙的产生机理具有重要意义。考虑到页岩气改造压裂级数多的特点,采用有限元方法,基于内聚力单元,构建了周期载荷作用下的水泥环界面密封性评价数值模型,研究了多级压裂过程中的水泥环界面胶结失效演变规律。研究结果表明:第一次循环结束后,后期循环加载过程中水泥环界面位移的累计增长程度较小,控制水泥环初次加载过程中的塑性变形可以提高维持水泥环密封完整性的压裂次数;降低水泥环弹性模量,适当提高水泥环的泊松比可以有效增加水泥环的长期密封能力,进而降低多级压裂过程中油气井环空带压的现象。研究成果可为页岩气井多级压裂过程中油气井环空带压的有效控制提供理论参考。

The phenomenon of sustained casing pressure is serious in the process of shale gas development in Sichuan,China,which seriously affects the efficient development of shale gas wells.Therefore,it is of great significance to accurately understand the generation mechanism of micro-annulus at cementing interface during multi-stage fracturing of shale gas wells.Considering that shale gas stimulation has many fracturing stages,a numerical model was biult up for sealing evaluation of cement sheath under cyclic loading by means of finite element method and based on cohesive element,the evolution law of interfacial cementation failure of cement sheath during multistage fracturing was studied.The results show that,after the first cycle,the cumulative increase of the interface displacement of the cement sheath was small in the later cyclic loading process,and controlling the plastic deformation of the cement sheath in the initial loading process can improve the fracturing times to maintain the seal integrity of the cement sheath.Reducing the elastic modulus of cement sheath and properly increasing its Poisson s ratio can effectively increase its long-term sealing ability,and then reduce the phenomenon of sustained casing pressure in oil and gas wells during multi-stage fracturing.The results can provide theoretical reference for the effective control of annular pressure in multi-stage fracturing of shale gas wells.